1. Field
The present disclosure relates to a photoelectric conversion element and a photoelectric conversion device including the photoelectric conversion element.
2. Description of the Related Art
Solar cells and photosensors (photodetectors) are example photoelectric conversion elements having a photoelectric conversion layer. Solar cells have been widely researched and developed using light in a broad wavelength range in order to improve photoelectric conversion efficiency. For example, there have been proposed solar cells in which electrons are photoexcited in two steps through a quantum level (including a superlattice miniband and an intermediate band) formed between the valence band and the conduction band of a matrix material, which makes it possible to use light having long wavelengths (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-509772; A. Marti', E. Antoli'n, C. R. Stanley, C. D. Farmer, N. Lo'pez, P. Diaz, E. Ca'novas, P. G. Linares, and A. Luque, “Production of Photocurrent due to Intermediate-to-Conduction-Band Transitions: A Demonstration of a Key Operating Principle of the Intermediate-Band Solar Cell,” PHYSICAL REVIEW LETTERS, PRL 97, 247701 (2006)).
Such solar cells including quantum dots are compound solar cells including quantum dot layers having quantum dots. When the quantum dots are present in the matrix semiconductor, two-step photoexcitation through a quantum level enables absorption of light in a wavelength region that has not been used to date (absorption of photons of energy less than the bandgap of the matrix material) and thus increases photocurrent.
Quantum dot photosensors having quantum dots have also been researched and developed to improve sensitivity. For example, there has been proposed a quantum dot photosensor that uses intersublevel transition through a quantum level in the conduction band to increase sensitivity in the middle- and far-infrared regions (Japanese Unexamined Patent Application Publication No. 2012-109434).
Current solar cells including quantum dot layers have insufficient photoelectric conversion efficiency. A reason for this may be the low efficiency of two-step light absorption through a quantum level (including a superlattice miniband and an intermediate band). In particular, such solar cells have a problem of low absorption associated with the intersublevel transition (intersubband transition), which is light absorption in the second step of the two-step light absorption process. Quantum dot photosensors also have low absorption associated with the intersublevel transition (intersubband transition) and thus have a low SN ratio. Consequently, anticipated high-sensitivity sensors have not been realized.
The intersublevel transition in quantum structure has polarization dependency. In the case of quantum wells, absorption of only light polarized in the growth direction of the quantum wells occurs. In the case of quantum dots, absorption of both light polarized in the growth direction of the quantum dots and light polarized in the in-plane direction perpendicular to the growth direction of the quantum dots occurs. Since normal quantum dots in the growth direction are small, absorption of light polarized in the growth direction occurs at higher energy than absorption of light polarized in the in-plane direction. In the case of solar cells and sensors using electric current extraction, carriers produced by absorption of light polarized in the growth direction are produced in a quantum level that is high in energy position, and thus the carriers can be extracted with low bias. However, light polarized in the growth direction is not efficiently obtained by incidence of light on the device in the growth direction. There is a need to deliver light at an oblique angle relative to the device or to introduce, for example, a textured structure or a diffraction grating into the device.
To improve the absorption, techniques for increasing the number of quantum dot layers are being actively developed. However, increasing the number of quantum dot layers is not practical because it imposes restrictions on materials such as small critical thickness and on crystal growth from the viewpoint of lattice mismatch and also increases crystal growth time. In the research step, a quantum dot structure having 300 layers has been reported.